In producing gears, shafts, pulleys, constant-velocity joints and so on to be used in various gear transmission devices such as automotive transmissions and differentials and structural parts such as crank shafts and con rods, it is a common practice to conduct forging or the like, followed by cutting into a final shape. Since the cutting cost accounts for a large part of the production cost, a steel constituting the structural parts as described above should has good machinability.
On the other hand, such a structural part as described above is molded into the final shape and then subjected to a surface hardening treatment, e.g., carburizing or carbonitriding (involving treatments under atmospheric pressure, reduced pressure, vacuum or plasma atmosphere), if necessary, followed by quenching/tempering, high-frequency quenching or the like to ensure a definite strength. However, the strength is sometimes lowered during such a treatment. In particular, there is a problem that the strength in the direction perpendicular to the rolling direction (which is commonly called the transverse direction) is liable to lower.
There has been known that lead (Pb) is an element capable of improving the machinability of a steel for machine and structural use without lowering the strength. Namely, Pb is an element which is highly effective for improving machinability. However, it has been pointed out that Pb is harmful to human body. Moreover, Pb causes additional troubles such as a lead fume in melting and a need for processing cutting dusts. Accordingly, there has been recently required a steel containing no Pb (Pb-free) and yet exhibiting good machinability.
As a technique for ensuring good machinability without adding Pb, there have been known steels in which the S content is increased up to about 0.06%. However, this technique suffers from a problem that the mechanical properties (toughness and fatigue strength) are frequently lowered and there is a limit to increase the S content. This is seemingly because a sulfide (MnS) expands long in the rolling direction and thus causes lowering in the toughness in the transverse direction. In a part which requires high strength, it is particularly necessary to reduce the S content as much as possible. From these viewpoints, it is required to establish a technique that good machinability can be further improved without positively adding Pb or S.
Under these circumstances, various techniques have been proposed to establish good machinability without positively adding Pb or S. In particular, studies have been made on the relationship between inclusions in the steel and machinability (see, for example, Patent Document 1). Further, various techniques have been proposed concerning the control of inclusions.
For example, Patent Document 1 discloses a technique of improving the machinability of a Ti-added high-strength steel by adding Ca in the presence of a definite amount of oxygen and Ti and thus allowing the coexistence of sulfides containing Ca and oxides containing Ca contributing to the improvement in machinability. Patent Document 2 discloses a steel for machine and structural use wherein the amount of sulfides containing Ca or oxides is controlled by adjusting Ca/Al ratio so as to give stable machinability with regulated variation in tool life.
Patent Document 3 or 4 discloses a technique of regulating variation in machinability by ensuring at least a specific area ratio of sulfides inclusion containing Ca in an amount of 0.3 to 40%, or by ensuring at least a specific count of sulfides containing Ca in an amount of 0.1 to 10%. Moreover, Patent Documents 5 and 6 disclose a technique of improving the machinability of a steel for machine and structural use by using an inclusion having a double structure which contains a core made of a oxide containing Ca and a surrounding area thereof made of a sulfide containing Ca.
Patent Document 7 discloses a technique of improving the machinability (in particular, chip-disposability and tool life) by lowering the melting point of oxides by adding Ca and refining sulfide inclusions by inhibiting the solute of Ca in sulfide inclusions (in particular, MnS) through the regulation of the steel making conditions.
Non-patent Document 1: 182th/183rd Nishiyama Kinen Gijutsu Koza, The Iron and Steel Institute of Japan, pp. 181 to 226, Kaizaibutsu Seigyo, Oct. 22, 2004 in Tokyo, November 12 in Kobe
Patent Document 1: JP-A-2005-272903
Patent Document 2: JP-A-2005-273000
Patent Document 3: JP-A-2000-34538
Patent Document 4: JP-A-2000-219936
Patent Document 5: JP-A-2003-55735
Patent Document 6: JP-A-2004-91886
Patent Document 7: JP-A-2003-213368